Nondiscoloring antioxidants for natural rubber and synthetic rubbers



Patented july 10, 1951 2,560,028 NONDISCOLORING ANTIOXIDANTS Fon NATURAL RUBBE RUBBERS Leland J. Kitchen and'. Joseph C.

R AND SYNTHETIC Ambelang,

Akron, Ohio, assignors to The Firestone Tire &

Rubber Company, of Ohio Akron, Ohio, a corporation No Drawing. Application August 6, 1947, Serial No. 766,872

14 Claims.

1 This invention relates to the stabilization of natural rubber and synthetic rubber by incorporation therein of a 2-alkyl-4-methylphenol in which the alkyl group contains seven or more carbon atoms up to, for example, twenty.

It has been found that the 4-methylphenois substituted in the 2-position by an alkyl group of one, two or three carbon atoms is not a good stabilizer. However, the 4-methylphenols substituted in the 2-position with alkyl groups containing seven or more carbon atoms, and particularly the octyl-substituted compounds and especially 2-tt-octyl-4-methylphenol, are good stabilizers and particularly valuable because they are nondiscoloring. Phenyl b e t a naphthylamine which is widely used commercially as a stabilizer, is known to be objectionable because it discolors white and other light-colored stocks, such as the stocks used for the white side walls of tires and light-colored mechanical goods. etc. The stabilizers of this invention have comparable stabilizing eflect and are superior in that they do not discolor. They retard and inhibit oxidative deterioration of natural rubber vulcanizate and retard and inhibit oxidative deterioration of both cured and uncured synthetic rubbers. When used with synthetic rubbers, they are advantageously added to the latex, particularly if the latex is obtained by emulsion copolymerization; and, thus,

they are effective in retarding decomposition during the drying of the latex as well as in subsequent treatments of the uncured copolymer and in the preparation and use of the vulcanizate.

The 2-alkyl-4-methylphenols which may be used in practicing the invention include particularly 2-tt-octyl-4-methyl-phenol (prepared by alkylating p-cresol with diisobutylene) and 2-secoctyl-4-methylpheno1 (prepared by alkylating p-cresol with l-octene) The 2-alkyl substituent is large, containing at least seven carbon atoms and includes both cycloaliphatic and aliphatic groups, such as, for example, the various heptyl, octyl, nonyl, etc., groups including aliphatic groups containing up to twenty carbon atoms and p-methylcyclohexyl, etc., including the cycloallphatic groups up to and including those containing twenty carbon atoms. The 2-alkyl substituent is large, containing at least 7 carbon atoms and includes both cyclic alkyl and openchain alkyl radicals, such as, for example, the various heptyl, octyl, nonyl, etc. radicals including alkyl groups containing up to 20 carbon atoms, and alkyl-substituted cyclic alkyl radicals, including p-methylcyclohexyl, etc.

STABILIZATION OF NATURAL RUBBER In the stabilization of natural rubber the invention finds particular importance in the preparation of white side walls for pneumatic tires. The stabilizers inhibit or retard weather checkill 2 ing and flex cracking without substantial discoloration or staining even on aging.

The stabilizers may be incorporated in the rubbar by milling or Banbury or they may be emulsified and the emulsions added to the rubber latex before coagulation. The following example is illustrative:

Natural rubber stock was compounded according to the following formula. all parts being by weight:

Parts Thick gristly crepe natural rubber 100 Wax 2 Ultramarine dye 0.1 Zinc oxide r; Titanium dioxide 20 Sulfur 3 Stearic acid 1.2 2-tt-octyl-4-methylphenol l Benzothiazyl disulfide 0.4 Amine activator 0.5

A control was obtained by milling stock according to the same formula but omitting the 2-ttoctyl-4-methylphenol. The two stocks were vul canized 20, 40 and 60 minutes at 280 F. Tapered strips stretched to 12 per cent elongation were exposed to Florida sunlight, both direct and indirect, for two months to determine to what extent the test stock would be discolored. In the tests neither the test stock nor the control underwent discoloration. Similar stocks containing antioxidants of the aromatic amine type discolored. For example, phenyl-beta-naphthylamine rendered the vulcanizate tan during the sunlight exposure test.

In another test for discoloration the test stock and control were exposed in a'weatherometer for four hours. Both the test and control stocks remained white during the test.

The stocks were then tested to note the effect of the stabilizer on their flex life. One-half inch dumbbell strips were flexed with 0 to per cent elongation until all strips were broken. The flex life was noted, and the cracks were counted to give the value recorded in the last column of the following table. The values in the table are averages of those obtained on 20, 40 and 60 minute cures.

TABLE I Natural rubber flex life Flex Life stabir Ill atc of crack lzer ormatxon Percent Hours Imnmve, Cracks/hour ment 2-tt-cctyl-4-methylphenol 49.72 37 2.3 None (c ntrol) 35.42 3.8

In another test, strips of the stocks were exposed to a sunlamp for thirty-six hours to determine the eilect of the stabilizer on weather checking. The strips or the: test stock showed less checking (formation of a network of surface 4 to set up more rapidly, the condition or set up after aging is an indication of antioxidant activity. In tests. this set up was measured by determining the time required to reverse the cured cracks) than the controls which contained no 5 condition by milling, as follows: The cold (water stabilizer. cooled) rolls (6-inch diameter) of a mill were En Iz set at a clearance of 0.005 inch. A 200-gram sheet i gg g gggg of aged copolymer was folded twice and passed once through the rolls. It was then milled until Tests were conducted to determine the efiect 10 t Sheet on the roller contained no holes The f the S rs 111 rubber-like D Y of time in seconds, exclusive of the initial pass, rethe type obtained from a conjugated l n and quired to bring the sheet to this condition is rea monomer having the formula CH2==C(R)--X m mm followin table; when R is hydrogen, methyl, ethyl, propyl or T m chlorine and X is -CN, CONH:, COR or -COOR' when R contains one to live carbon Oomlymer set up atoms. In the first series oi tests the copolymer used was produced from 1,3-butadiene and 32 Time, per cent acrylonitrile. The stabilizer was 2-tt- Ammndampmnt octyl-4-methylphenol and was added as an emulsion in sodium oleate solution to the latex obg-ggggiyggggggglgg :2 tained from emulsion copolymerization of the emomyusgiso coni i honoi'iIIIIIIIIIIIIIIIIIIII 2o monomers. The latex was coagulated with a Mammy pure grade of aluminum sulfate. The coagulum'. after washing on a wash mill, was dried for 25 the hy phen l, which is twenty hours at 70 C. the only one of the above compounds which The above dried copolymer prepared from latex 601115 Within the Scope of this en on, was containing 2 per cent (based on the dryweight P e! e equ val nt of phenyl-beta-naphof the copolymer) oi the stabilizer was compared y amme in inhibiting set up of the copolymer with copolymer identically obtained but to which during g g- The results obtained with 2 per t phenyl-beta-naphfl ylamjne had been the other two alkylated phenols recorded in the added instead 01' the stabilizer of this invention. above table indicate that a ylation other than The control thus produced was light lavenderallwlaflon f a para-crawl in he -po ition can brown after yi whereas t t t material result in marked lessening of the antioxidant efwas white. A third sample of copolymer, identifectiveness- 3 mefllyl i r -b tylphenol cally prepared, using a like amount of sodium used because it has the same molecular oleate but no stabilizer, was light tan in color and welflht as the best mfltflial, and e P- W partially resiniiled on the surface after drying. P1181101 was used 11609-118! it Williams an Octy Thus, drying in t absence. 0f stabilizer causes group. It is evident that the latter two samples deterioration, and the stabilizers or this inven- 40 of the above table underwent a r ter degree tion prevent such deterioration when used for of Set up than the test and 00111101 S pl drying at any elevated temperature. th t e imaged and the aged copolymers con- The following table shows the results'of aging t 8 -tt-octyl-4-methy1p and P e y copolymer in a circulating air oven maintained phthylamine (control) were compounded at 95 0. Samples of 180 grams and 225 square in carbon black Sh s according to h followcentlmeters in area were tested. The aging was i m la conducted for ten days. After such aging, the Parts properties or the two samples were a represented C0p 1ymer+29t stabilizer 100 in the following table: ghal'iiialcid 3.5

o e 5 n Carbon black 40 Butadiene-acrvlonitrile copolvmer aging m 2 Accelerator 1.3 Antioxidant Present 6353;, 33:33,? The accelerator used was N-cyclohexyl-Z-benzothiazwl suli'enamide. 2-tt-octyl-4-methyl henol light very good. The Sto we v vanlzed sixty minutes at phenyl-beta-napht ylamine(control) dark brown. Do. 374 11 Th physical properties of the cured stocks are recorded in the following table:

Tun: IV Physical properties of vulcanizate MM Mt fifij, Jedi P. I. i. P. e. 5. Percent i-tt-octyl-i-methzl 515 2,515- coo phenyl-beta'nng hmme. 625 2,825 700 i-tt-oetyl-i-met lana- 815 3,5 610 pheiiyl-bota-nap ylammd 850 3, 575 700 A phenomenon encountered with butadienederived synthetic rubbers which precedes deterioration, during agin is a .set up or condition or mild cure caused by cross-link formation between polymer chains. Since this set up precedes The stabilizer of this invention likewise was effective in imparting age-resistance to a vlucanizate prepared from a butadiene-acrylonitrile copolymer. A vulcanizate stabilized with 2-tt-octyld-methylphenol was subjected to air-bomb deterioration and the less stable polymers tend aging-air at 60 p. s. i. for ten hours at 250 1".

Physical properties of the vulcanizate are recorded in the following table.

The aged vuicanizate was of good quality.

STABILIZATION OF BUTADIENE-STYRENE COPOLYMER The stabilizers have been found to be eilective and non-discoloring when used in a copolymer of a conjugated diene and an aromatic vinyl compound. For instance, 2 per cent of 2-tt-octyl-4- methylphenol emulsified in sodium oleate was added to a rubber-like copolymer of 1,3-butadiene and styrene containing 25 per cent of combined styrene. A control was prepared in which phenylbeta-naphthylamine was similarly used. On drying, the test material remained white in color; whereas the control turned brown.

The test sample and control were then further tested against a blank which contained no stabilizer and a fourth sample which contained a trialkylated phenol. These four samples were aged by placing them in a circulating air oven where they were kept at 110 C. for two days. The fol- In the second column of the table a rating of poor indicates deterioration had set in, as evidenced by stiflening and resinification of the sample. Thus, it is evident that the 2-tt-octyl-4- methylphenol is an eflective antioxidant for such copolymer, providing considerably better protection than the trialkylated phenol of the same molecular weight.

Thus, the stabilizers of this invention are effective in natural rubber and synthetic rubbers. They may be used in the stabilization of other rubbers than those mentioned, such a polymers of isoprene, 1,3-butadiene and substituted 1,3- butadienes, such as chloroprene. They may be used in copolymers prepared with 1,3-butadiene, such as copolymers of 1,3-butadiene and other conjugated dieneswith a vinyl aromatic compound, such as, for example, styrene, alphamethyl-styrene, nuclear-substituted styrenes, monochlorostyrene, dichlorostyrene, vinylnaphthalene, vinylbiphenyl, vinylcarbazole, 2-vinyl-5- ethylpyridine, 2-ethyl-5-vinylpyridine, etc. The conjugated dienes which may be employed to produce rubber-like copolymers include, as is wellknown, for example, 1,3-butadiene, isoprene, 2-

rated monomeric units, including butadiene polymerizates.

What we claim is:

1. Uncured, rubber copolymer of butadiene and acrylonitrile stabilized with a small amout of 2-ttoctyl-4-methylphenol.

2. Cured, natural rubber stabilized with a small amount of 2-tt-octyl-4-methylphenol.

3. Cured, rubber copolymer of butadiene and acrylonitrile stabilized with a small amount of 2- tt-octyl-4-methylphenol.

4. The method of preparing rubber copolymer of butadiene and acrylonitrile which comprises mixing with a latex thereof a small amount of 2- tt-octyl-4-methylphenol, as stabilizer, coagulating, and drying the coagulum at an elevated temperature.

5. The method of curing natural rubber which comprises heating it while it has mixed therewith sulfur and, as stabilizer, a small amount of 2-ttoctyl-4-methylphenol. 1

6. The method of curing rubber copolymer of butadiene and acrylonitrile which comprises heating it while it has mixed therewith sulfur and, as stabilizer, a small amount of 2-tt-octyl-4-methylphenol.

7. A composition containing rubber material from the group consisting of natural rubber and rubber copolymer of butadiene and acrylonitrile, said material being stabilized with a small amount of a 2-octyl-4-methylphenol.

8. The method of curing a rubber material from the group consisting of natural rubber and copolymer of butadiene and acrylonitrile, which comprises heating the same while it has mixed therewith sulfur, and a small amount of a 2- oetyl-4 -methylphenol.

9. Uncured, rubber copolymer of butadiene and acrylonitrile stabilized with a small amount of a 2-octyl-4-methylphenol.

10. Cured, rubber copolymer of butadiene and acrylonitrile stabilized with a small amount of a 2-octyl-4-methylphenol.

11. Cured, natural rubber stabilized with a 45 small amount of a 2-octy1-4-methylphenol.

12. The methodof curing natural rubber which comprises heating it while it has mixed therewith sulfur and, as stabilizer, a small amount or 2- octyl-4-methylphenol.

60 13. The method of curing rubber copolymer of butadiene and acrylonitrile which comprises heating it while it has-mixed therewith sulfur and, as stabilizer, a small amount of a 2-octyl-4-methylphenol.

14. The method of preparing rubber copolymer of butadiene and acrylonitrile which comprises mixing with a latex thereof a small amount of a 2-octyl-4-methylphenol, as stabilizer, coagulating, and drying the coagulum at an elevated 60 temperature.

LELAND J. KITCHEN. JOSEPH C. AMBELANG.

REFERENCES CITED The following references are of record in the me of this patent:

- UNITED STATES PATENTS Number Name Date Re. 20,337 Stein Apr. 20, 1937 2,335,039 Sibley Nov. 23, 1943 2,351,347 Luten June 13, 1944 2,356,929 Hart Aug. 29, 1944 2,370,756 Sibley Mar. 6, 1945 2,471,887 Nelson May 31, 1949"" 

7. A COMPOSITION CONTAINING RUBBER MATERIAL FROM THE GROUP CONSISTING OF NATURAL RUBBER AND RUBBER COPOLYMER OF BUTADIENE AND ACRYLONITRILE, SAID MATERIAL BEING STABILIZED WITH A SMALL AMOUNT OF A 2-OCTYL-4-METHYLPHENOL. 